Magnetic Random Access Memory (MRAM) is a non-volatile computer memory technology based on magnetoresistance. MRAM is non-volatile and so can maintain memory content when an MRAM memory device is not powered. MRAM data is stored by magnetoresistive elements. Generally, the magnetoresistive elements in an MRAM cell are made from two magnetic regions, each of which accepts and sustains magnetization. The magnetic field of one region (the “fixed region”) is fixed in its magnetic orientation, and the magnetic orientation of the other region (the “free region”) can be changed during operation. Thus, a programming current can cause the magnetic orientations of the two magnetic regions to be either parallel, giving a lower electrical resistance across the magnetoresistive elements (which may be defined as a “0” state), or anti-parallel (i.e., directed oppositely, e.g., 180 degrees, from the parallel orientation), giving a higher electrical resistance across the magnetoresistive elements (which may be defined as a “1” state) of the MRAM cell. The switching of the magnetic orientation of the free region and the resulting high or low resistance states across the magnetoresistive elements enables the write and read operations of the typical MRAM cell.
One type of MRAM cell is a spin torque transfer MRAM (STT-MRAM) cell. A conventional STT-MRAM cell may include a magnetic cell core, which may include a magnetic tunnel junction (MTJ), or may include a spin valve structure. An MTJ is a magnetoresistive data storing element including two magnetic regions (one fixed and one free) and a non-magnetic, electrically insulating region in between, which may be accessed through data lines (e.g., bit lines), access lines (e.g., word lines), and an access transistor. A spin valve has a structure similar to the MTJ, except a spin valve employs a non-magnetic, electrically conductive region between the two magnetic regions.
In operation, a programming current may be caused to flow through the access transistor and the magnetic cell core. The fixed region within the cell core polarizes the electron spin of the programming current, and torque is created as the spin-polarized current passes through the core. The spin-polarized electron current interacts with the free region by exerting a torque on the free region. When the torque of the spin-polarized electron current passing through the core is greater than a critical switching current density (Jc) of the free region, the torque exerted by the spin-polarized electron current is sufficient to switch the direction of the magnetization of the free region. Thus, the programming current can be used to cause the magnetic orientation of the free region to be aligned either parallel to or anti-parallel to the orientation of the fixed region, and, when the magnetic orientation of the free region is switched between parallel and anti-parallel, the resistance state across the core is changed.
The free regions and fixed regions of conventional STT-MRAM cells exhibit magnetic orientations that are horizontal, also known as “in-plane,” with the width of the regions. Efforts have been made to form perpendicularly oriented (“out-of-plane”) STT-MRAM cells in which the fixed regions and the free regions exhibit vertical magnetic orientations (also known in the art as perpendicular magnetizations). However, finding and implementing suitable materials and designs for the cell core and forming the cell core structure has been a challenge. For example, as illustrated in FIG. 1, in forming a magnetic cell core of a conventional STT-MRAM cell with PMA, materials may be formed over a substrate 110. The materials may include a conductive material 120, optional intermediary materials 130, magnetic materials 140, a non-magnetic material 150, additional optional intermediary materials 160, and a hard mask material 170, the combination of materials together forming a precursor structure 100, as illustrated in FIG. 1. The hard mask material 170, to be used in a subsequent patterning process, may be a sacrificial, non-conductive material. With reference to FIG. 2, one or more of the magnetic materials 140 within the precursor structure 100 may exhibit a vertical magnetic orientation 180 upon initial formation.
The magnetic materials 140 exhibiting the vertical magnetic orientation 180 may also be characterized by a strength of the magnetic materials' 140 perpendicular magnetic anisotropy (“PMA”). The strength (also referred to herein as the “magnetic strength” or the “PMA strength”) is an indication of the magnetic materials' 140 resistance to alteration of the magnetic orientation. A magnetic material exhibiting a vertical magnetic orientation 180 with a high magnetic strength may be less prone to alteration of its magnetic orientation out of the vertical alignment than a magnetic material exhibiting a vertical magnetic orientation 180 with a lower magnetic strength.
Following formation of the precursor structure 100 (FIG. 1), with reference to FIG. 3, the precursor structure 100 (FIG. 1) may thereafter be patterned, e.g., etched, to form a cell core structure 300. Ideally, the cell core structure 300 may have a structure defining sidewalls indicated by dashed lines 302, with essentially vertical sidewalls along the etched conductive material 120, intermediary materials 130, 160, magnetic materials 140, non-magnetic material 150, and hard mask material 170. However, conventional material formation processes may result in one or more of the formed materials 120, 130, 140, 150, 160, 170 experiencing a lateral, residual tensile stress or lateral, compressive stress upon initial formation of the precursor structure 100, and conventional patterning processes used to form the cell core structure 300 may cause responses to these stresses in a resulting strain, manifested in lateral contraction or expansion of the respective materials 120, 130, 140, 150, 160, 170 as illustrated by oblique sidewalls 304.
The strain exhibited by lateral material expansion, or, in some circumstances, contraction, of the previously-stressed precursor structure 100 (FIG. 1) may deteriorate the magnetic strength of one or more of the magnetic materials 140, e.g., in the free region, in the fixed region, or in both the free region and the fixed region. For example, the magnetic strength may decrease, increasing the risk that the magnetic material's 140 magnetic orientation 180 may be unintentionally shifted out of the vertical orientation. In extreme cases, therefore, the magnetic strength may deteriorate to such an extent that the magnetic orientation 180 shifts out of vertical, as illustrated in FIG. 4. Consequently, conventional processes for fabricating STT-MRAM cells with perpendicular magnetizations (also referred to herein as “vertical magnetic orientations”), may adversely impact, e.g., deteriorate, the net magnetic strength, i.e., the strength of the PMA, or even alter the direction, e.g., alignment, of the magnetic orientation 180, compared to the magnetic strength and orientation of the magnetic materials prior to patterning. The reduced magnetic strength or, in extreme circumstances, altered magnetic orientation may lower the energy barrier and weaken bit thermal stability and may adversely affect data retention in the resulting STT-MRAM cell.